Lens made of a crystalline material

ABSTRACT

As a preliminary stage in manufacturing a lens or lens part for an objective, in particular a projection objective for a microlithography projection system, an optical blank is made from a crystal material. As a first step in manufacturing the optical blank, one determines the orientation of a first crystallographic direction that is defined in the crystallographic structure of the material. The material is then machined into an optical blank so that the first crystallographic direction is substantially perpendicular to an optical blank surface of the optical blank. Subsequently, a marking is applied to the optical blank or to a mounting element of the optical blank. The marking has a defined relationship to a second crystallographic direction which is oriented at a non-zero angle relative to the first crystallographic direction.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Patent ApplicationSerial No. PCT/EP02/12690 filed Nov. 13, 2002, which, in turn, claimsthe priority of International Patent Application Serial No.PCT/EP02/05050 filed May 8, 2002, both of which are hereby incorporatedherein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing an optical blankfrom a crystalline material, as well as to an optical blank. The opticalblank serves as a preliminary stage in the production of a lens or alens part. Consequently, the invention also relates to a method ofproducing a lens or a lens part from a crystalline material, as well asto a lens or lens part. Lenses or lens parts of the kind that theinvention relates to are used in optical objectives, specifically inprojection objectives for a microlithography projection system.Consequently, the invention also relates to objectives, specificallyprojection objectives for a microlithography projection system.

Methods for producing an optical blank from a fluoride crystal materialhave been disclosed in U.S. Pat. No. 6,201,634. The optical blank isused to make lenses for a projection objective for a microlithographyprojection system. The lens axes of the lenses are oriented withpreference in the crystallographic <111>-direction. According to U.S.Pat. No. 6,201,634, the crystallographic <111>-direction is selected forthe purpose of minimizing the detrimental effects of stress-inducedbirefringence.

It is a general trait of birefringent lenses that a non-polarized lightray is split into two rays with, respectively, different states ofpolarization, different speeds of propagation, and different directions.When used in an objective, birefringent lenses will cause a loss inoptical resolution unless appropriate corrective measures are taken. Thebirefringent effect in lenses can be caused, for example, bystress-induced birefringence that occurs as a result of themanufacturing process or as a result of mechanical forces acting on thelens. The phenomenon of birefringence is of particular importance incrystal optics. Anisotropic crystals are birefringent. However,isotropic crystals, too, such as the cubic fluoride crystals, exhibit anintrinsic birefringence, which becomes particularly noticeable atwavelengths in the far ultraviolet range (<200 nm). Cubic fluoridecrystals such as calcium fluoride and barium fluoride are preferred lensmaterials for projection objectives with a working wavelength in thisrange. Consequently, in view of its detrimental effect at wavelengths inthe far ultraviolet range, the intrinsic birefringence of these crystalsneeds to be compensated by appropriate measures.

In the present context, it is essential to have a clear system ofnotations for the crystallographic directions. Following are thenotations by which crystallographic directions, crystallographic planes,and lenses whose lens axes are aligned in a specific crystallographicdirection will hereinafter be characterized.

The indices for the crystallographic directions will hereinafter bebracketed between the symbols “<” and “>”, and the indices for thecrystallographic planes will be bracketed between the symbols “{” and“}”. The crystallographic directions are perpendicular to thecorrespondingly indexed crystallographic planes. For example, thecrystallographic direction <100> is perpendicular to thecrystallographic plane {100}. Crystals with a cubic lattice structure,which includes the fluoride crystals that are of interest in the presentcontext, have the principal crystallographic directions <110>,<{overscore (1)}10>, <1{overscore (1)}0>, <{overscore (1)}{overscore(1)}0>, <101>, <10{overscore (1)}>, <{overscore (1)}01>, <{overscore(1)}0{overscore (1)}>, <011>, <0{overscore (1)}1>, <01{overscore (1)}>,<0{overscore (1)}{overscore (1)}>, <111>, <{overscore (1)}{overscore(1)}{overscore (1)}>, <{overscore (1)}{overscore (1)}1>, <{overscore(1)}1{overscore (1)}>, <1{overscore (1)}{overscore (1)}>, <{overscore(1)}11>, <1{overscore (1)}1>, <11{overscore (1)}>, <100>, <010>, <001>,<{overscore (1)}00>, <0{overscore (1)}0>, and <00{overscore (1)}>.

Because of the symmetries of cubic crystals, the principalcrystallographic directions <100>, <010>, <001>, <{overscore (1)}00>,<0{overscore (1)}0>, and <00{overscore (1)}> are equivalent to eachother. Therefore, those crystallographic directions that are orientedalong one of the principal directions <100>, <010>, <001>, <{overscore(1)}00>, <0{overscore (1)}0>, and <00{overscore (1)}> will hereinafterbe identified by the prefix “(100)-”, and crystallographic planes thatare perpendicular to these directions will also be identified by thesame prefix “(100)-”.

Furthermore, the principal directions <110>, <{overscore (1)}10>,<1{overscore (1)}0>, <{overscore (1)}{overscore (1)}0>, <101>,<10{overscore (1)}>, <{overscore (1)}01>, <{overscore (1)}0{overscore(1)}>, <011>, <0{overscore (1)}1>, <01{overscore (1)}>, and <0{overscore(1)}{overscore (1)}> are likewise equivalent to each other. Therefore,those crystallographic directions that are oriented along one of thelatter group of principal directions will hereinafter be identified bythe prefix “(110)-”, and crystallographic planes that are perpendicularto these directions will also be identified by the same prefix “(110)-”.

Finally, the principal directions <111>, <{overscore (1)}{overscore(1)}{overscore (1)}>, <{overscore (1)}{overscore (1)}1>, <{overscore(1)}1{overscore (1)}>, <1{overscore (1)}{overscore (1)}>, <{overscore(1)}11>, <1{overscore (1)}1>, <11{overscore (1)}> are likewiseequivalent to each other. Therefore, those crystallographic directionsthat are oriented along one of the latter group of principal directionswill hereinafter be identified by the prefix “(111)-”, andcrystallographic planes that are perpendicular to these directions willalso be identified by the same prefix “(111)-”.

Any statements made hereinafter in regard to one of the aforementionedprincipal crystallographic directions should be understood to be equallyapplicable to the equivalent principal crystallographic directions.

As is known from the article “Intrinsic Birefringence in CalciumFluoride and Barium Fluoride” by J. Burnett et al. (Physical Review B,Volume 64 (2001), pp. 241102-1 to 241102-4), lenses made of calciumfluoride crystal material or barium fluoride crystal material exhibitintrinsic birefringence. The intrinsic birefringence is in this casestrongly dependent on the material orientation of the fluoride crystallens and on the light ray direction. The effect is maximal for a raythat passes through a lens along the crystallographic (110)-direction.The measurements presented by Burnett et al. demonstrate that a lightray traveling in the crystallographic (110)-direction of a calciumfluoride crystal is subject to a birefringence that amounts to 11.8±0.4nm/cm at a wavelength of λ=156.1 nm, to 3.6±0.2 nm/cm at a wavelength ofλ=193.09 nm, and to 0.55±0.07 nm/cm at a wavelength of λ=253.65 nm. Onthe other hand, with a light propagation in the <100> direction or inthe <111> direction of the crystal, no intrinsic birefringence occurs incalcium fluoride, as is also predicted by theory. Thus, the intrinsicbirefringence has a strong directional dependence and increasessignificantly for shorter wavelengths.

The directional dependence of the intrinsic birefringence in a fluoridecrystal with a cubic crystal structure is shown in the published article“The trouble with calcium fluoride” by J. Burnett et al. (spie'soemagazine, March 2002, pp. 23-25 and FIG. 4), which may be accessed at“http://oemagazine.com/fromTheMagazine/marO2/biref.html”. The intrinsicbirefringence of a light ray depends in this case on the aperture angleas well as on the azimuth angle of a light ray. As is made evident inFIG. 4, the intrinsic birefringence has a fourfold azimuthal symmetry ifthe lens axis is oriented in the crystallographic (100)-direction, athreefold azimuthal symmetry if the lens axis is oriented in thecrystallographic (111)-direction, and a twofold azimuthal symmetry ifthe lens axis is oriented in the crystallographic (110)-direction. Byrotating two fluoride crystal lenses relative to each other about theirlens axes, it is possible to reduce the detrimental influence of theintrinsic birefringence. Favorable results are obtained with an angle ofrotation of 45° for two lenses whose lens axes are oriented in thecrystallographic (100)-direction, with an angle of rotation of 60° fortwo lenses whose lens axes are oriented in the crystallographic(111)-direction, and with an angle of rotation of 90° for two lenseswhose lens axes are oriented in the crystallographic (110)-direction. Bysimultaneously using respective pairs of (100)-lenses, (111)-lenses, and(110)-lenses, it is possible to reduce the optical path differencebetween two mutually orthogonal states of polarization. As a furtherpossibility, using calcium fluoride lenses and barium fluoride lenses incombination likewise results in a compensation of the detrimentalinfluence of the intrinsic birefringence because, according to FIG. 2 ofthe same article, the respective birefringence effects for correspondingcrystallographic directions in barium fluoride and calcium fluoride haveopposite signs.

Projection objectives and microlithography projection systems have beendisclosed, e.g., in the Patent Application Publication WO 01/50171 A1(U.S. Ser. No. 10/177,580) and the references cited therein. Theexamples of embodiments presented in that patent application are purelyrefractive as well as catadioptric projection objectives with numericalaperture values of 0.8 and 0.9 at working wavelengths of 193 nm as wellas 157 nm. The material used for the lenses is calcium fluoride.

The not pre-published patent application PCT/EP 02/05050 (U.S. Ser. No.10/367,989) by the same applicant includes a description of differentcompensation methods for reducing the detrimental influence of theintrinsic birefringence, e.g., in the objectives that are presented asexamples in WO 01/50171 A1 (U.S. Ser. No. 10/177,580). Among otherpossibilities, the solutions disclosed therein include the parallel useof (100)-lenses with (111)-lenses or (110)-lenses of the same fluoridecrystal material as well as the use of compensation coatings. Thedisclosures of PCT/EP 02/05050 (U.S. Ser. No. 10/367,989) and of WO01/50171 A1 (U.S. Ser. No. 10/177,580) are hereby incorporated herein inits entirety.

To conclude, the compensation methods described above for reducing thedetrimental influence of the birefringence are based among other thingson the use of lenses that are rotated relative to each other about theirlens axes. The angle of rotation between two lenses depends for exampleon the crystallographic direction in which the lens axis is oriented.For example in lenses made by a method according to the previously citedreference U.S. Pat. No. 6,201,634, the lens axes are oriented in thecrystallographic (111)-direction. Based on what has been said above, afavorable result in reducing the detrimental influence of the intrinsicbirefringence is obtained in this case with an angle of rotation of 600between two (111)-lenses. The angle of rotation is defined in relationto the crystallographic structures of the two lenses. However, theoutward appearance of a lens gives no indication of its crystallographicstructure.

OBJECT OF THE INVENTION

The invention therefore has the purpose to propose a method of producingoptical blanks from a crystalline material as a preliminary stage forthe production of lenses or lens parts, so that when the lenses and lensparts made from the blanks are subsequently installed in objectives, themethod provides the capability to orient the lenses and lens parts withtheir crystal structures rotated by a given angle relative to eachother.

SUMMARY OF THE INVENTION

To meet the purpose of the invention, i.e., to provide the capability ofsetting a predetermined angle of rotation in an optical objectivebetween a crystallographically defined direction of a lens or lens partand a reference direction of the objective, or between respectivecrystallographically defined directions of two lenses or lens parts, itis of advantage if each lens or lens part or its lens mount carries amarking that stands in a defined relationship with the crystallographicstructure of the lens or lens part.

The term “lens parts” refers for example to individual lenses which areseamlessly joined through the technique of wringing to form one singlelens. In a general sense, lens parts are the components of a singlelens.

Preferred crystals used as raw materials for the optical blanks are thecubic fluoride crystals which include, e.g., calcium fluoride, bariumfluoride, or strontium fluoride.

A multitude of shaping and surface-finishing process steps are requiredto bring a lens or lens part into its final form. For lenses or lensparts that consist of a crystalline material, the raw material isnormally a mono-crystalline block or a mono-crystalline ingot which canfor example be produced through one of the methods described in thepreviously mentioned U.S. Pat. No. 6,201,634. Starting from themono-crystalline block, one proceeds to produce an optical blank, forexample by sawing or grinding. The term optical blank means apreliminary stage of a lens or lens part. One or more lenses or lensparts can be manufactured from the optical blank. If two or more lensesor lens parts are produced from one optical blank, the latter is carvedup into individual optical blanks by sawing. In a next work phase, theindividual optical blanks are ground and/or polished, so thatmeasurements can be performed at the surfaces that have been prefinishedin this manner. The optical blanks resulting from the foregoing processhave the form of individual material disks of cylindrical shape.

It is advantageous if the optical blank has an optical blank surfacewhose normal vector is aligned in a first crystallographic directionthat is defined within the crystallographic structure. As anadvantageous choice, this first crystallographic direction is aprincipal crystallographic direction, for example the crystallographic<100>-, <111>-, or <110>-direction.

As a first step of the inventive method, it is therefore necessary todetermine the orientation of the first crystallographic direction in theoptical blank. This determination can be made on the optical blankbefore the latter is cut apart further into individual smaller opticalblanks. It is likewise possible, to first cut apart the large blank andto make the determination on the individual smaller blanks.

As a second step of the method, the optical blank is shaped by sawingand grinding operations in such a manner that the first crystallographicdirection is approximately perpendicular to the optical blank surface.The deviation between the first crystallographic direction and thenormal vector of the optical blank surface is preferably smaller than5°. The optical blank surface represents in this case the front or backof the material disk.

As a third step, a marking is applied to the optical blank or to itsmounting element. The marking has a defined relationship to a secondcrystallographic direction that is oriented at a non-zero angle relativeto the first crystallographic direction. The second crystallographicdirection can likewise be a principal crystallographic direction, or itcan be another direction that is defined within the crystallographicstructure, for example the crystallographic <331>-direction or thecrystallographic <511>-direction.

The marking can consist for example of a point- or line marking that isengraved on the cylindrical border surface of the blank or on themounting element that is rigidly connected to the optical blank. Themounting element can consist of a metallic, ceramic, or glass-ceramicmaterial.

The defined relationship between the second crystallographic directionand the marking can be established by defining the marking as an indexmark to indicate a reference direction that is perpendicular to thefirst crystallographic direction and represents a projection of thesecond crystallographic direction into a plane whose normal vector isoriented along the first crystallographic direction. In a cylindricaloptical blank with a symmetry axis pointing substantially in the firstcrystallographic direction, the reference direction is preferablydefined by a radial line that intersects the symmetry axis. Thus, themarking indicates the intersection of the reference direction with thecylindrical outside border of the optical blank or with the mountingelement. Accordingly, the marking also defines an azimuth angle of theprojected second crystallographic direction in relation to a coordinatesystem that is connected to the optical blank. The azimuth angle isdefined as the angle between the reference direction and a coordinateaxis that is perpendicular to the symmetry axis and intersects thesymmetry axis.

To determine the first crystallographic direction, the optical blank canbe exposed to a test radiation, in particular an X-ray test radiationfrom a defined incident direction. The test radiation is reflected atthe crystallographic planes that are associated with the firstcrystallographic direction, for example the crystallographic{111}-planes, whereby a Bragg reflex pattern is produced. Since thewavelength of the test radiation and the material of the optical blankare known, the theoretical angle of the incident and reflected testradiation relative to the first crystallographic direction is a knownquantity determined by Bragg's law. To find the first crystallographicdirection, one proceeds to make continuing adjustments to the positionof the optical blank relative to the Bragg measurement apparatus untilthe Bragg reflex for the first crystallographic direction is detected.Based on the orientations of the measurement apparatus and the opticalblank relative to each other, one determines the orientation of thefirst crystallographic direction relative to the normal vector of theoptical blank surface of the optical blank. If the normal vector of theoptical blank surface does not coincide with the first crystallographicdirection, the optical blank is reworked, e.g., by grinding, until theangle deviation is less than ±5°.

In an advantageous embodiment, the optical blank is rotatably supportedrelative to an axis that is perpendicular to the optical blank surfaceof the optical blank. With this arrangement, the Bragg reflexes aredetermined for different angles of rotation, in the simplest case at 0°and 90°.

The reference direction can likewise be determined by analyzing a Braggreflex. In this case, the test radiation is reflected at thecrystallographic planes that are associated with the secondcrystallographic direction.

Alternatively, the orientation of the reference direction can bedetermined through the Laue method.

It is of advantage to set a rule for selecting the reference directionso that the birefringence will cause, e.g., a maximum of the opticalpath difference for two mutually orthogonal states of linearpolarization in a light ray traversing the lens, if the projection ofthe light ray into a plane that is perpendicular to the firstcrystallographic direction runs parallel to the reference direction. Ifthe compensation methods of the foregoing description are used, i.e.,the concept of rotating lenses relative to each other, the prescribedangles of rotation can easily be set based on this rule for selectingand marking the reference direction. It is also possible to adopt therule for selecting and marking the reference direction so that theoptical path difference takes on a minimum value in a light raytraversing the lens if the projection of the light ray into a plane thatis perpendicular to the first crystallographic direction runs parallelto the reference direction.

If the first crystallographic direction runs in the <100>-direction orin the <111>-direction or a direction that is equivalent to either ofthese crystallographic directions, it is advantageous if the projectionof the second crystallographic direction into a plane that is orthogonalto the first crystallographic direction runs parallel to the projectionof the <110>-direction or a <110>-equivalent direction into the sameplane. What makes this choice of orientation advantageous is the factthat the optical path difference is maximal for light rays that runparallel to the <110>-direction or a <110>-equivalent direction.

If the first crystallographic direction is oriented in the<111>-direction or a <111>-equivalent crystallographic direction, it isadvantageous if the second crystallographic direction is oriented in the<331>-direction or a <331>-equivalent crystallographic direction.

If the first crystallographic direction is oriented in the<100>-direction or a <100>-equivalent crystallographic direction, it isadvantageous if the second crystallographic direction is oriented in the<511>-direction or a <511>-equivalent crystallographic direction.

Because the test radiation that is used for the determination of theBragg reflexes can damage the material at the optical blank surfaces, itis advantageous if the parts of the optical blank that have beentraversed by the test radiation are removed by grinding or polishing.

The foregoing method is advantageously used to produce an optical blankas the initial product stage from which a lens or a lens part for anobjective is manufactured.

In the production of a lens or lens part from a blank that has beenprepared according to the foregoing description, the optical surfaces ofthe lens or lens part are machined in such a way that the lens axis endsup aligned approximately parallel to the first crystallographic axis,i.e., approximately parallel to the normal vector of the optical blanksurface. The deviation between the first crystallographic direction andthe lens axis should preferably be less than ±5°. The curved lenssurfaces of the lens are produced by grinding and polishing the opticalsurfaces of the optical blank. If the lens surfaces are rotationallysymmetric, the lens axis is represented by the symmetry axis. If thelens surfaces are not rotationally symmetric, the lens axis can berepresented by the central ray of an incident bundle of light rays or bya straight line in relation to which the ray angles of all light raysinside the lens take on minimal values. The term lens in this contextincludes for example refractive or diffractive lenses as well ascorrection plates with free-form corrective surfaces. Planar-parallelplates, too, are considered as lenses if they are arranged in the lightpath of an objective. The lens axis of a planar-parallel plate is inthis case perpendicular to the planar lens surfaces.

If the marking of the reference direction that has been applied to theoptical blank is obliterated by the subsequent manufacturing process ofthe lens or lens part, it is important to transfer the marking of thereference direction to the lens or lens part or to its mounting element.

In lenses or lens parts that are used in high-performance opticalsystems such as projection objectives for applications inmicrolithography, the angular deviation between the lens axis and thefirst crystallographic direction has an effect even if it is smallerthan 5°. It is therefore advantageous to determine this angulardeviation very accurately. This can be accomplished for example throughX-ray diffractometry methods. It is further advantageous to know notonly the magnitude of the angle, but also the orientation of the firstcrystallographic axis. The orientation can be described in terms of adirectional vector of the angular deviation of the lens axis. Thedirectional vector of the angular deviation is perpendicular to the lensaxis and represents the projection of the first crystallographicdirection into a plane that is perpendicular to the lens axis. Thedirection of the angular deviation of the lens axis is marked on thelens or lens part, for example on the border of the lens. Alternatively,the marking can also be applied to a mounting element of the lens orlens part. If the lens or lens part or its mounting element alreadycarries a marking for the reference direction, it is also possible todetermine the angle between the reference direction and the direction ofthe angular deviation, with a positive or negative sign indicating thesense of rotation, and to assign the result as a unit-specific value tothe respective lens or lens part. For example, the value for this anglecould be stored in a database in which the material data andmanufacturing data for the lens or lens part are kept.

As an alternative, the marking for the second crystallographic directioncan also be applied to the lens or lens part after the latter has beenproduced from an optical blank of a crystal material. The lens is madefrom a blank, e.g., by grinding and polishing of the lens surfaces. Inthis process, the surfaces are formed in such a manner that the lensaxis ends up parallel to a first crystallographic direction thatpreferably coincides with a principal crystallographic direction. In anext step, the lens or lens part or its mounting element is marked withan index marking which has a defined relationship to a secondcrystallographic direction enclosing a non-zero angle with the firstcrystallographic direction. The second crystallographic direction canlikewise coincide with a principal crystallographic direction, or it canbe another crystallographic direction defined within thecrystallographic structure, for example the crystallographic<331>-direction if the lens axis is oriented in the crystallographic<111>-direction, or the crystallographic <511>-direction if the lensaxis is oriented in the crystallographic <100>-direction.

The marking can consist for example of a point- or line marking that isengraved on the cylindrical border surface of the lens or lens part oron the mounting element that is rigidly connected to the lens or lenspart. The mounting element can consist of a metallic, ceramic, orglass-ceramic material.

The defined relationship between the second crystallographic directionand the marking can be established by defining the marking as an indexmark to indicate a reference direction that is perpendicular to thefirst crystallographic direction and represents a projection of thesecond crystallographic direction into a plane whose normal vector isoriented in the direction of the lens axis. The reference direction ispreferably defined by a radial line that intersects the lens axis. Thus,the marking indicates for example the intersection of the referencedirection with the cylindrical outside border of the lens or lens partor with the mounting element. Accordingly, the marking also defines anazimuth angle of the projected second crystallographic direction inrelation to a coordinate system that is connected to the lens or lenspart.

The same methods as were proposed above for an optical blank can also beused for determining the reference direction in a lens or lens part. Forthe measurement of the Bragg reflex it is advantageous if the positionof the lens is adjustable so that the test radiation meets the curvedlens surface at a defined point of incidence. Particularly ifmeasurements are made at different rotated positions of the lens, it isadvantageous if the test radiation meets the lens in the area of thelens vertex.

In order to avoid self-shading in the case of concave lens surfaces, itis advantageous to select the second crystallographic direction in sucha manner that the incident test radiation and the reflected radiationthat is used for the determination of the first crystallographicdirection and of the reference direction is not disturbed by the lensgeometry.

Crystal materials that are advantageously used in objectives at shortwavelengths of less than 200 nm are the cubic fluoride crystals such ascalcium fluoride, barium fluoride, or strontium fluoride.

Only at wavelengths shorter than 200 nm does the intrinsic birefringencein cubic fluoride crystals manifest itself strongly enough to makeappropriate corrective measures necessary. Therefore, the determinationof the reference direction and the in some cases required determinationof the direction of the angular deviation of the lens axis are ofadvantage primarily for applications involving wavelengths shorter than200 nm. Lenses and lens parts that carry a marking of a referencedirection and in some cases a direction of the angular deviation of thelens axis are used with preference for objectives in which thedetrimental effect of birefringence is reduced by rotating the lenses inrelation to each other. The marking which is correlated to thecrystallographic orientation significantly simplifies the setting of atargeted angle of rotation between individual lenses. Based on thebirefringent properties of fluoride crystals which follow theoreticalprediction and based on the known methods of compensation, the angles ofrotation between the individual lenses or lens parts of an objective canbe determined so that the detrimental effects of birefringence on theimaging performance of an objective are significantly reduced.

It is particularly advantageous in the process of determining the anglesof rotation, if the lens-specific values of the angle between the firstcrystallographic direction and the lens axis as well as the direction ofthe angular deviation of the lens axis are taken into account.

Determining and marking only the direction of the angular deviation isadvantageous if the optical performance characteristics depend primarilyon the angular deviation between the first crystallographic directionand the lens axis. Through an appropriate rotation of the lens about itslens axis by a predetermined amount, it is possible to achieve acorrective effect on the imaging properties of the objective bycombining several lenses or lens parts that are rotated relative to eachother. This allows lenses or lens parts to be used even if they have anangular deviation between the first crystallographic direction and thelens axis. The difficulties in the manufacture of lenses or lens partsof crystalline material are thereby alleviated to a considerable extent,since the production tolerances can be relaxed.

The objective in the aforementioned case can be a purely refractiveprojection objective consisting of a multitude of lenses arranged withrotational symmetry relative to the optical axis, or the objective canbe a projection objective of the catadioptric type.

Projection objectives of this kind can be used advantageously inprojection systems that include a light source, an illumination system,a mask-positioning system, a mask that carries a structure, a projectionobjective, a substrate-positioning system, and a light-sensitivesubstrate.

This microlithography projection system finds application in themanufacture of microstructured semiconductor elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained hereinafter in more detail withreference to the drawings, wherein:

FIG. 1 schematically represents a cross-section of an optical blank;

FIG. 2 schematically represents a plan view of the optical blank of FIG.1;

FIG. 3 schematically represents a cross-section of a lens held in amounting element;

FIG. 4 schematically represents a plan view of the lens with themounting element of FIG. 3;

FIG. 5 schematically represents a cross-section of a further example ofa lens;

FIG. 6 schematically represents a plan view of the lens of FIG. 5;

FIG. 7 schematically represents an objective in a perspective view;

FIG. 8 represents a lens section of a projection objective; and

FIG. 9 schematically represents a projection system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As an example that embodies the invention, the following is adescription of the manufacturing process of calcium fluoride lenseswhose lens axes coincide substantially with the crystallographic<111>-direction. However, the methods described herein can also beapplied to the manufacture of lenses of other crystalline materials witha cubic structure such as for example barium fluoride or strontiumfluoride. Furthermore, the lens axes can also be aligned in thecrystallographic <100>- or <110>-direction. The method is suitable forthe manufacture of planar-parallel lenses as well as lenses or lensparts with curved surfaces.

In a preliminary stage of the manufacturing process of a lens, anoptical blank is produced. FIGS. 1 and 2 schematically illustrate anoptical blank 1 which was produced by the method according to theinvention. FIG. 1 represents a cross-section of an optical blank 1 alongthe line A-A which is indicated in the plan view drawing of FIG. 2.

In a first step of the method, the orientation of the crystallographic<111>-direction 3 of the optical blank 1, in this case a calciumfluoride disk, is determined. The crystallographic <111>-direction 3 isperpendicular to the crystallographic {111}-planes 5, a few of which areindicated in FIG. 1. This determination can be made with a high degreeof accuracy through crystallographic methods such as, e.g., by adetermination of cleaving surfaces or by generating etching craters.Better results in the determination of crystallographic directions areobtained with X-ray diffractometry. A suitable instrument for thistechnique is a goniometer arrangement used with monochromatic X-rays.The occurrence of a Bragg reflex for the {111}-planes 5 is determinedwith the help of tabulated literature values. The tabulated values showthe required angles of incidence for the different reflex indices. Toperform the measurement, the calcium fluoride disk is rotated about anaxis that is perpendicular to the calcium fluoride disk. As a result,one obtains the deviation of the <111>-direction from the normal vectorof the calcium fluoride disk for different angles of rotation. It isadvantageous if the deviation is determined for at least two rotarypositions. In the present example, the measurements are made at 0° and90°. Additional measurements can be performed at 180° and 270° or atfurther angles in between to enhance the measuring accuracy.

In a second step, the calcium fluoride disk is worked into a shape wherethe normal vector of the calcium fluoride disk is parallel to thecrystallographic <111>-direction 3, so that the crystallographic<111>-direction 3 is substantially perpendicular to the optical blanksurface 7. The deviation measured in the first step serves as a basisfor a controlled correction, i.e., a specifically defined reshaping ofthe calcium fluoride disk by sawing or grinding. Following thisprocessing step, the normal vector of the calcium fluoride disk isoriented in the crystallographic <111>-direction within a tolerance ofless than 5°.

As a third step, a reference direction 9 is determined in the calciumfluoride disk, such that the reference direction has a definedrelationship to a further crystallographic direction. If the normalvector of the calcium fluoride disk is oriented in the <111>-direction3, it is advantageous if one of the three crystallographic directions<110>, <011> and <101> or one of the directions <100>, <010> and <001>is known, which are grouped in threefold symmetry relative to the<111>-direction. This is of interest because the intrinsic birefringencecauses a maximum optical path difference in a light ray for two mutuallyorthogonal states of linear polarization, if the light ray travels inthe <110>-direction or a <110>-equivalent direction in a calciumfluoride lens. No optical path difference occurs if the light raytravels in the <100>-direction or a <100>-equivalent direction. Each ofthe three crystallographic directions <110>, <011> and <101> is angledat 35° to the <111>-direction, and each of the directions <100>, <010>and <001> is angled at 55° to the <111>-direction. For reasons that havea physical explanation, the X-ray reflections of (110)- or (100)-planesare not measurable in crystals with a calcium fluoride structure. It istherefore necessary to use the Bragg reflexes of other crystallographicplanes that have a defined relationship to the (100)- and (110)-planes.For example, it is possible to use a (331)-Bragg reflex. Each of thethree crystallographic directions <331>, <133> and <313> runs at anangle of 22° to the <111>-direction. The crystallographic<331>-direction 11 is indicated in FIG. 1. It runs perpendicular to thecrystallographic {331}-planes 13, some or which are indicated in thedrawing. The (331) Bragg reflex for monochromatic copper Kα-radiation(8048 eV) in calcium fluoride is found at 38°. Thus, the reflex is foundwith an angle of incidence of 16° and a detector angle of 60° relativeto the reference plane that is defined by the surface 7 of the calciumfluoride disk. In the course of a 360°-rotation of the disk about itsnormal vector axis, Bragg reflexes will be observed at three angularpositions. Each of the Bragg reflexes indicates a position where one ofthe directional vectors of the three targeted (331)-planes lies in theplane of incidence of the Bragg measurement. The projections of thethree (331)-directions onto the disk surface 7 are parallel to theprojections of the three crystallographic directions <110>, <011> and<101>. Thus, by determining the crystallographic directions <331>, <133>and <313>, the directions of the projections of the <110>-, <011>- and<101>-directions are determined at the same time. If the normal vectorof the disk surface deviates from the <111>-direction, the positionsettings for the source and the detector have to be adjustedaccordingly.

The reference direction 9 in FIG. 2 is aligned with the projection ofthe crystallographic <331>-direction into a plane that is orthogonal tothe crystallographic <111>-direction. The reference direction 9 isindicated by a radial line which intersects the symmetry axis 17 of theoptical blank 1.

Alternatively, the crystallographic orientations can also be determinedfrom a Laue pattern. In contrast to the measurements of Bragg reflexesof monochromatic X-ray light which have been described above, the Lauemethod works with “white” light, i.e., X-ray light with a broad band ofwavelengths. With white X-ray light, one obtains Bragg reflexes ofdifferent families of crystallographic planes generating a Laue patternthat is characteristic for the material. If the <111>-direction isparallel to the direction of the incident light, a Laue pattern ofthreefold symmetry is produced. If the <111>-direction deviates by a fewdegrees from the normal vector of the disc, the pattern will be slightlydistorted. The exact analysis of the Laue pattern, e.g. with anappropriate software program, can be used to determine the deviation ofthe <111>-direction from the normal vector of the disc. By evaluatingthe pattern, it is further possible to identify the triplets ofcrystallographic directions <110>, <011>, <101> or <100>, <010>, <001>and thereby determine the orientation of the disc.

In a fourth step, at least one marking 15 is applied to the opticalblank 1 to indicate the reference direction 9. Thus, the marking 15 hasa defined relationship to the crystallographic <331>-direction 11. Themarking can be made, e.g., by engraving, etching, or with a writinginstrument. The cylindrical border of the optical blank 1 suggestsitself as a natural location for applying the marking. Alternatively,the marking can also be applied on a mounting element that has a fixedconnection to the optical blank 1.

In a fifth step, a lens is produced from the optical blank 1. FIGS. 3and 4 schematically illustrate the lens 31 produced from the opticalblank 1. The lens 31 is held in a mounting element 33. FIG. 3 shows themounted lens 31 in a sectional view along the line B-B which isindicated in the plan view drawing of FIG. 4.

The lens 31 is made in such a way that the lens axis 35 ends up parallelto the <111>-direction of the crystal structure of the lens. Thisprocessing step does not destroy the marking 15 that was applied in themanufacturing process of the optical blank 1, because many machiningoperations such as grinding and polishing apply only to the top andbottom of the lens but not to the cylindrical circumference. However, ifthe circumference is to be machined also, for example in a turningoperation, the marking will have to be transferred with sufficientaccuracy to the mounting device of the calcium fluoride disc andreapplied to the cylindrical border after the machining operation hasbeen completed.

An additional marking 37 for the reference direction 9 is applied to themounting element 33.

In a further example, a lens is manufactured from an optical blank ofcubic fluoride crystal material, for example calcium fluoride, where theblank has been manufactured in such a way that its crystallographic<111>-direction is already substantially perpendicular to the surface ofthe optical blank. In this case, the marking is applied only after thelens has been manufactured.

In a first step, the lens is made out of the optical blank in such amanner that the lens axis is oriented in the <111>-direction.

The reference direction is determined as a next-following step. Theprocedures used for this determination are the same as described abovefor the calcium fluoride disc. However, it is important to preciselyadjust the height of the point of incidence of the X-ray on the lenssurface. The support surface for the lens is thereforeheight-adjustable. This makes it possible to follow the curved profileof the lens if different points on the curved lens surface are to bemeasured. It further needs to be noted that the curvature can cause ashade-out of the incident or outgoing ray. Shade-outs can be avoided byselecting a suitable Bragg reflex in combination with the appropriategeometrical arrangement for the measurements.

In the case of planar-parallel plates, the foregoing procedure with agoniometer arrangement can be used at any point of the surface.

In the machining process of the optical blanks and lenses, it needs tobe taken into account that the irradiation of calcium fluoride withX-rays can lead to the formation of color centers. The penetration depthof copper-Kα-radiation in calcium fluoride is approximately 30 μm. Inorder to avoid the possible existence of color centers in the material,it is advantageous if the X-ray analysis is performed only on calciumfluoride blanks or lenses in which a sufficient amount of surfacematerial will be removed in subsequent processing steps. In the case ofa Cu—Kα-irradiation, this means that the surface material should beremoved to a depth of at least 30 μm.

FIGS. 5 and 6 schematically illustrate a lens 51 according as a furtherexample that embodies the invention. FIG. 5 shows the lens incross-section along the line C-C which is indicated in the plan viewdrawing of FIG. 6.

The calcium fluoride lens 51 in the example of FIGS. 5 and 6 is not a(111)-lens, but a (100)-lens. However, the lens axis 53 is not alignedexactly in the crystallographic <100>-direction 55; there is an angulardeviation 6 between the lens axis 53 and the crystallographic<100>-direction 55. The crystallographic <100>-direction 55 isperpendicular to the crystallographic {100}-planes 57.

In addition to the magnitude of the angle δ, it is also important toknow the direction 63 of the angular deviation of the lens axis. Theazimuthal direction 63 of the angular deviation is obtained as theprojection of the crystallographic <100>-direction 55 into a plane thatis perpendicular to the lens axis 53. The direction 63 of the angulardeviation is preferably constituted by a radial line that intersects thelens axis 53. A marking 65 is applied to the lens 51 to indicate thedirection 63 of the angular deviation. The marking can also be appliedto a mounting device which is not illustrated in FIGS. 5 and 6. Themarking shown in FIG. 6 is located at the intersection of the linerepresenting the direction 63 with the cylindrical outside border of thelens 51.

The orientation of the crystallographic <100>-direction 55 in relationto the lens axis 53 can be determined by analyzing the respective Braggreflexes of the crystallographic <100>-direction 55 for differentrotated positions of the lens 51. To perform this process, the lens 51is rotated about its lens axis 53. It is advantageous to determine thedeviation in at least two rotated positions of the lens. In thisexample, the measurements are performed at 0° and 90°. To achieve anincreased accuracy, additional measurements can be made at 180° and270°.

As an alternative, the deviation of the lens axis 53 from thecrystallographic <100>-direction 55 can also be determined by using theLaue method, in which case the incident test radiation is oriented inthe direction of the lens axis.

In addition to the marking 65, the lens 51 also carries the marking 67.The marking 67 has a defined relationship to the crystallographic<511>-direction 59 which is perpendicular to the crystallographic{511}-planes 61. The marking is at the intersection of a radial linerepresenting the reference direction 69 with the cylindrical outsideborder of the lens 51. The reference direction 69 is obtained as theprojection of the crystallographic <511>-direction 59 into a plane thatis perpendicular to the lens axis 53. The line that indicates thereference direction 69 intersects the lens axis 53. The reason forselecting the crystallographic <511>-direction 59 is that the projectionof the <511>-direction 59 into a plane that is perpendicular to the lensaxis 53 runs parallel to the projection of the crystallographic <011>direction into the same plane. The <011>-direction, in turn, isdistinguished among other directions in the crystal, because the opticalpath difference for two mutually orthogonal states of polarization dueto the intrinsic birefringence is maximal for a light ray travelingthrough the crystal in the crystallographic <011>-direction.

A single marking is sufficient for the purpose of setting the angle ofrotation of a lens about its lens axis relative to a fixed referencedirection of the objective. Since the lens 51 has the marking 67 for thereference direction 69, one could omit the additional marking 65 for thedirection 63 of the lens axis deviation and determine instead the anglebetween the reference direction 69 and the direction 63 of the angulardeviation of the lens axis as a characteristic value associated witheach lens. For example, the value for the direction of the angulardeviation could be stored together with the deviation angle in adatabase in which, e.g., the material- and production data of the lens51 are stored. Thus, the angle as well as the direction of the lens axisdeviation is available for the optimization procedures.

FIG. 7 schematically illustrates a first embodiment of an objective 71according to the invention. The objective produces an image IM of anobject OB. Shown in the drawing are the lenses 73, 75, 77 and 79. Thelens axes of the lenses 73, 75, 77 and 79 are aligned in the directionof the optical axis OA. The lenses 73, 75 are (111)-lenses and thelenses 77, 79 are (100)-lenses of calcium fluoride. To compensate forthe detrimental influence of the intrinsic birefringence, the lenses arearranged with a rotation relative to each other about the lens axis sothat the difference between the respective optical path differences fortwo orthogonal states of polarization in an outermost aperture ray 81and a ray traveling along the optical axis OA is minimized. Meeting thiscondition requires an angle of rotation of 60° between the (111)-lenses73 and 75. In accordance with the invention, setting the angle ofrotation is a simple procedure, as the lenses 73 and 75 carry themarkings 83 and 85 indicating the respective reference directions 87 and89. The reference directions 87 and 89 represent the projections of thecrystallographic <331>-directions of the lenses into planes that runperpendicular to the respective lens axes. The angle of rotation betweenthe (100)-lenses 77 and 79 is not exactly 45°, because thecrystallographic <100>-directions in these lenses are not alignedexactly in the directions of the respective lens axes. The respectivedirections 95 and 97 of the angular deviation of the lens axis areindicated by the markings 91 and 93. The magnitude and orientation ofthe lens axis deviation are taken into account in the optimization ofthe angle of rotation between the lenses 77 and 79. After calculatingthe optimal angle of rotation between the lenses 77 and 79, it is asimple procedure to set the lenses at the calculated angle with the helpof the markings 99 and 101. The latter markings indicate the referencedirections 103 and 105 which represent the projections of thecrystallographic <511>-directions into planes that run perpendicular tothe respective lens axes of the lenses 77 and 79.

A method of optimization will now be described which serves to determineon the one hand the lens axis orientation of the individual lenses inspecific principal directions of the crystal structure and on the otherhand the angles of rotation between the lenses in an objective of aknown optical design. Several lenses of the objective consist of abirefringent fluoride crystal material, with the birefringent propertiesof the lenses representing likewise a known quantity, meaning forexample that the influence of the intrinsic birefringence on a light raycan be theoretically predicted as a function of the aperture angle andthe azimuth angle if the crystal material as well as its materialorientation relative to the coordinate system of a lens are known.However, the birefringent properties may also be known from measurementsthat were made on the lenses. With the birefringent properties of thelenses and the optical design of the objective being known, the opticalpath difference for two mutually orthogonal states of linearpolarization that occurs in a light ray inside the objective is likewiseknown. In the following process, the optical path difference occurringin a light ray represents the quantity that is to be optimized, meaningthat its absolute value is to be minimized. Analogously, theoptimization can also be extended to an entire bundle of individuallight rays. Possible degrees of freedom that are available for thisoptimization are the angles of rotation of the individual lensesrelative to each other and the orientation of the lens axes in relationto the principal crystallographic directions. In accordance with theprinciples described above, it is advantageous if on the one hand, thelens axes are oriented in the principal crystallographic directions andon the other hand, the angles of rotation relative to each other take ononly certain discrete values that depend on the lens-axis orientation ofthe respective lens.

Three degrees of freedom are available for the orientation of the lensaxis, i.e., the lens axes can be oriented in the (100)- , (111)- or(110)-direction of the crystallographic structure.

Lenses whose lens axes are oriented in the same principalcrystallographic direction or in equivalent crystallographic directionsare combined in individual groups, where each group has at least twolenses.

The discrete angles of rotation of the lenses of a group depend on theorientation of the lens axis.

If a group has a number n of (100)-lenses, the angles of rotation are tobe specified as${\gamma = {\frac{90{^\circ}}{n} + {{{m \cdot 90}{^\circ}} \pm {10{^\circ}}}}},$where m means an arbitrary integer. Accordingly, if the group iscomposed of two (100)-lenses, the angle of rotation between the twolenses is ideally 45° or 135°, 225° . . .

If a group has a number n of (111)-lenses, the angles of rotation are tobe specified as${\gamma = {\frac{120{^\circ}}{n} + {{{m \cdot 120}{^\circ}} \pm {10{^\circ}}}}},$where m means an arbitrary integer.

If a group has a number n of (110)-lenses, the angles of rotation are tobe specified as${\gamma = {\frac{180{^\circ}}{n} + {{{m \cdot 180}{^\circ}} \pm {10{^\circ}}}}},$where m means an arbitrary integer.

Thus, the discrete angles of rotation of the lenses relative to eachother and the discrete crystallographic orientations of the lenses areavailable as degrees of freedom.

Within this parameter universe, one faces the task of finding thespecific combination of angles of rotation and crystallographicorientations for the individual lenses where the optimization quantitytakes on a minimum value, or finding a combination where theoptimization quantity falls below a given threshold value.

For every objective, there is an optimal solution where the optical pathdifferences for two mutually orthogonal states of linear polarizationfor an entire bundle of light rays take on minimal values.

However, it is an extremely large undertaking to solve the problem offinding the true optimum, particularly if the objective has a largenumber of lenses, as is the case for the objective 8 of FIG. 8. FIG. 8represents the lens section of a catadioptric projection objective 8 forthe wavelength of 157 nm. The optical data for this objective are listedin Table 1. This example is borrowed from the patent application WO01/50171 A1 (U.S. patent application Ser. No. 10/177,580), where thesame objective is shown in FIG. 9 and specified in Table 8. For adetailed functional description of the objective, the reader is referredto the patent application WO 01/50171 A1 (U.S. patent application Ser.No. 10/177,580). All lenses of this objective consist of calciumfluoride crystal. The numerical aperture on the image side of theobjective is 0.8.

There are optimization methods available that may not necessarilydeliver the true optimum, but will at least lead to a solution that isadequate in view of the practical application that the objective isintended for. A closely related problem in the mathematical literatureis known as “the problem of the traveling salesman” where the shortestpossible route has to be found to visit a given set of cities on ageographical map.

The optimization may be accomplished by one of the following methods,which are known from the literature under the names:

-   -   1. Monte Carlo search,    -   2. Simulated annealing    -   3. Threshold accepting    -   4. Simulated annealing with intermediate reheating    -   5. Genetic algorithm

In a first embodiment of the method outlined above for compensating thedetrimental effect of intrinsic birefringence, there are four degrees offreedom (DOF) available for each lens:

-   -   DOF 1: (111)-lens with angle of rotation 0°    -   DOF 2: (111)-lens with angle of rotation 60°    -   DOF 3: (100)-lens with angle of rotation 0°    -   DOF 4: (100)-lens with angle of rotation 45°        The angles of rotation are defined in relation to a fixed        reference direction in the object plane O.

In the case of the projection objective 8 of FIG. 8, the Monte Carlosearch method was used with the four given degrees of freedom DOF1 toDOF4 to find the optimum combination of the crystallographic lens-axisorientations and of the angles of rotation β_(L) of the lenses L801 toL817 relative to a fixed reference direction in the object plane O. Thecrystallographic orientations of the lens axes and the angles ofrotation β_(L) for the lenses L801 to L817 are listed in Table 2. Alsoshown for each lens is the optical path difference for two mutuallyorthogonal states of polarization for the highest and lowest outermostaperture rays. The two outermost aperture rays originate from an objectpoint in the center of the object field, and their respective angleswith the optical axis OA in the image plane O′ correspond to theimage-side numerical aperture. The maximum optical path difference forthe objective as a whole is 5 nm. TABLE 2 Lens data for the objective ofFIG. 8 Optical path Optical path Orientation Angle of difference for thedifference for the of lens rotation β_(L) highest outermost lowestoutermost Lens axis [°] aperture ray [nm] aperture ray [nm] L801 <100>45 0.0 −3.1 L802 <111> 60 −13.0 29.7 L803 <100> 0 −15.1 −27.6 L803 <100>0 −26.0 −19.2 L802 <111> 60 28.3 −14.2 L804 <111> 0 −7.6 9.8 L805 <100>45 3.1 −1.0 L806 <100> 0 0.0 −2.1 L807 <111> 60 7.8 1.0 L808 <100> 450.0 −1.1 L809 <100> 0 0.0 −0.7 L810 <100> 0 −0.1 −1.5 L811 <100> 0 −3.9−1.7 L812 <111> 0 15.4 −5.0 L813 <100> 0 −3.7 −0.2 L814 <100> 0 −2.1−0.1 L815 <100> 45 −11.4 −6.6 L816 <111> 60 −16.8 49.6 L817 <111> 0 55.7−12.2 sum −5.0 −2.7

Additional degrees of freedom for the optimization are obtained byassigning the lenses to individual groups. The lens axes of the lenseswithin a group are oriented in the same principal crystallographicdirection. The lenses within a group are arranged with a rotationrelative to each other, such that the resulting distribution of theoptical path differences caused by the group for two mutually orthogonalstates of linear polarization is close to rotational symmetry. Thegroups themselves can now be rotated at arbitrary angles to each other,which represent additional degrees of freedom that can be used tocorrect aberrations that may for example be due to the manufacturingprocess.

In the embodiment of Table 2, the lenses L801 and L814 are assigned to afirst group of (100)-lenses. The two lenses are arranged with a 45°rotation relative to each other.

The lenses L802, L804, L807 and L812 are assigned to a second group with(111)-lenses and are divided into two subgroups, i.e., a subgroup withthe lenses L802, L807 and another subgroup with the lenses L804, L812.The lenses of a subgroup are not rotated relative to each other, or atmost they may be arranged at an angle of γ=1.120°±10°, where 1represents an integer. The two subgroups are arranged with a rotation60° relative to each other, so that the angle between two lenses ofdifferent subgroups is γ=60°+m·120°±10°, where m represents an integer.

The lenses L803, L805 and L815 are assigned to a third group with(100)-lenses and are divided into two subgroups, i.e., a subgroup withthe lens L803 and another subgroup with the lenses L805, L815. Thelenses of a subgroup are not rotated relative to each other, or at mostthey may be set at an angle of γ=1.90°±10°, where 1 represents aninteger. The two subgroups are arranged with a rotation 45° relative toeach other, so that the angle between two lenses of different subgroupsis γ=45°+m·90°±10°, where m represents an integer.

The lenses L808, L809 and L811 are assigned to a fourth group with(100)-lenses and are divided into two subgroups, i.e., a subgroup withthe lens L808 and another subgroup with the lenses L809, L811. Thelenses of a subgroup are not rotated relative to each other, or at mostthey may be arranged at an angle of γ=1.19°±10°, where 1 represents aninteger. The two subgroups are arranged with a rotation 45° relative toeach other, so that the angle between two lenses of different subgroupsis γ=45°+m·90°±10°, where m represents an integer.

The lenses L816 und L817 are assigned to a fifth group with(111)-lenses, where the two lenses are arranged with a rotation of 60°relative to each other.

In a second embodiment, there are eight degrees of freedom available foreach lens:

-   -   DOF 1: (111)-lens with angle of rotation 0°    -   DOF 2: (111)-lens with angle of rotation 60°    -   DOF 3: (100)-lens with angle of rotation 0°    -   DOF 4: (100)-lens with angle of rotation 45°    -   DOF 5: (110)-lens with angle of rotation 0°    -   DOF 6: (110)-lens with angle of rotation 90°    -   DOF 7: (110)-lens with angle of rotation 45°    -   DOF 8: (110)-lens with angle of rotation 135°

The result of the optimization process improves with the number ofdegrees of freedom, but the volume of the task expands exponentially.Further degrees of freedom are available by using a smaller step sizebetween the discrete angles of rotation.

Of course, the optimization can also be performed with angles ofrotation that have a smaller step increment between the discrete values.

It is further possible to also consider measurement data related tostress-induced birefringence, surface shape data of the lenses ormirrors and/or inhomogeneities in the lens material. In this manner, allpossible factors that interfere with the performance of the objectiveare taken into account, and the available degrees of freedom are used tofind a parameter constellation for the objective which will result in agood overall image quality.

In particular, if the direction of the angular deviation of the lensaxis is marked and if the magnitude as well as the orientation of thelens axis deviation from the respective principal crystallographicdirection are known for every lens of the objective, it is possible totake the effects caused by the angular deviation into account in theoptimization procedure. In lenses where the lens axes are orientedexactly in the crystallographic (100)-, (111)- or (110)-direction, thereare always equivalent angles of rotation due to the symmetry of thecrystallographic structure, such as for example γ=45°+m·90° for two(100)-lenses. If there is a deviation in the two lenses between the lensaxis and the respective principal crystallographic direction, one canuse the integer m as a degree of freedom in the optimization. Theinteger m can have one of the values 1, 2 and 3. Since the direction ofthe angular deviation as well as the reference direction is indicated bymarkings, the angle of rotation that is thus determined can be setexactly.

Following is a description of the individual steps of the optimizationprocess:

-   -   As a first step, a target function is calculated for an        objective in which the birefringent properties of the lenses are        known. The target function represents a measure for the        detrimental effect of the birefringence. For example, the        optical path difference in an outermost aperture ray for two        mutually orthogonal states of linear polarization can serve as a        target function. Another possibility is to define the target        function as the maximum or mean value of a distribution of        optical path differences of a bundle of light rays. The angles        of rotation and the crystallographic orientations as well as the        value of the target function for the current parameter        constellation of the objective are stored in memory. A threshold        value is prescribed for the target function, meaning that the        detrimental effect of the birefringence is tolerable if the        target function falls below the threshold.    -   As a second step, the target function is evaluated as to whether        it falls below the threshold value. If the target function is        found below the threshold value, the process is terminated. If        the target function fails to meet the threshold criterion, the        process continues with step three.    -   In the third step, the angles of rotation of the lenses relative        to each other and the crystallographic orientations of the        lenses are changed in the objective within the constraints of        the given degrees of freedom, using one of the aforementioned        methods, for example the Monte Carlo method.    -   Following the third step, the process loops back to step one,        keeping count of the number of loops completed. If the number of        loops exceeds a given maximum limit, the process is likewise        terminated.

According to this cycle of steps, the process is terminated if eitherthe target function falls below a given threshold or a maximum number ofloops has been exceeded. If the maximum number of loops is exceeded, theresult could be presented, e.g., in the form of a ranking list for theparameter constellations that were tried out and the values of thetarget function that were found for each constellation.

FIG. 9 illustrates in principle the arrangement of a microlithographyprojection system. The projection system 111 has a light source 113, anillumination device 115, a structure-carrying mask 117, a projectionobjective 119, and a substrate 121 that is exposed to the projection.The illumination device 115 collects the light of the light source 113,for example a KrF- or ArF laser, depending on the operating wavelength,and illuminates the mask 117. In conformance with the requirements ofthe exposure process, the illumination provides a specified degree ofhomogeneity of the light distribution at the mask and a specifiedillumination of the entry pupil of the objective 119. The mask 117 isheld in the light path by means of a mask holder 123. The mask 117 is ofa type used in microlithography applications, carrying a structure withdetails in the micrometer to nanometer range. As an alternative to aso-called reticle, the structure-carrying mask can also be constitutedby a controllable micro-mirror array or a programmable LCD array. Theprojection objective 119 forms an image of the mask 117, or in somecases of a part of the mask 117, on the substrate 121 that is held inposition by a substrate holder 125. The projection objective 119 is forexample the catadioptric objective represented in FIG. 8. The individuallenses 127 of the projection objective are arranged with a rotationrelative to each other in order to minimize the detrimental influence ofbirefringence or other phenomena. With the markings according to theinvention, the setting of the angle of rotation of the lenses is asimple procedure. The substrate 121 is typically a silicon wafercarrying a light-sensitive coating, the so-called resist. In furthersteps, the exposed substrates are subsequently processed intosemiconductor components. TABLE 1 L61 REFRACTIVE INDEX ½ FREE LENS RADIITHICKNESSES MATERIAL AT 157.629 nm DIAMETER 0 0.000000000 34.0000000001.00000000 82.150 0.000000000 0.100000000 1.00000000 87.654 L801276.724757380 40.000000000 CaF2 1.55970990 90.112 1413.944109416AS95.000000000 1.00000000 89.442 SP1 0.000000000 11.000000000 1.0000000090.034 0.000000000 433.237005445 1.00000000 90.104 L802 −195.92433638417.295305525 CaF2 1.55970990 92.746 −467.658808527 40.8411124681.00000000 98.732 L803 −241.385736441 15.977235467 CaF2 1.55970990105.512 −857.211727400AS 21.649331094 1.00000000 118.786 SP2 0.0000000000.000010000 1.00000000 139.325 253.074839896 21.649331094 1.00000000119.350 L803′ 857.211727400AS 15.977235467 CaF2 1.55970990 118.986241.385736441 40.841112468 1.00000000 108.546 L802′ 467.65880852717.295305525 CaF2 1.55970990 102.615 195.924336384 419.9813571651.00000000 95.689 SP3 0.000000000 6.255658280 1.00000000 76.3700.000000000 42.609155219 1.00000000 76.064 Z1 0.000000000 67.4495471151.00000000 73.981 L804 432.544479547 37.784311058 CaF2 1.55970990 90.274−522.188532471 113.756133662 1.00000000 92.507 L805 −263.16760572533.768525968 CaF2 1.55970990 100.053 −291.940616829AS 14.5365914241.00000000 106.516 L806 589.642961222AS 20.449887046 CaF2 1.55970990110.482 −5539.698828792 443.944079795 1.00000000 110.523 L807221.780582003 9.000000000 CaF2 1.55970990 108.311 153.07144306422.790060084 1.00000000 104.062 L808 309.446967518 38.542735318 CaF21.55970990 104.062 −2660.227900099 0.100022286 1.00000000 104.098 L80923655.354584194 12.899131182 CaF2 1.55970990 104.054 −1473.1892131769.318886362 1.00000000 103.931 L810 −652.136459374 16.359499814 CaF21.55970990 103.644 −446.489459129 0.100000000 1.00000000 103.877 L811174.593507050 25.900313780 CaF2 1.55970990 99.267 392.239615259AS14.064505431 1.00000000 96.610 0.000000000 2.045119392 1.00000000 96.552L812 7497.306838492 16.759051656 CaF2 1.55970990 96.383 318.2108317118.891640764 1.00000000 94.998 L813 428.724465129 41.295806263 CaF21.55970990 95.548 3290.097860119AS 7.377912006 1.00000000 95.040 L814721.012739719 33.927118706 CaF2 1.55970990 95.443 −272.6508723536.871397517 1.00000000 95.207 L815 131.257556743 38.826450065 CaF21.55970990 81.345 632.112566477AS 4.409527396 1.00000000 74.847 L816342.127616157AS 37.346293509 CaF2 1.55970990 70.394 449.2610787444.859754445 1.00000000 54.895 L817 144.034814702 34.792179308 CaF21.55970990 48.040 −751.263321098AS 11.999872684 1.00000000 33.475 0′0.000000000 0.000127776 1.00000000 16.430 ASPHERICAL CONSTANTS Asphereof lens L801 Asphere of lens L803 Asphere of lens L803′ K   0.0000 K  0.0000 K   0.0000 C1   4.90231706e−009 C1 −5.33460884e−009 C1  5.33460884e−009 C2   3.08634889e−014 C2   9.73867225e−014 C2−9.73867225e−014 C3 −9.53005325e−019 C3 −3.28422058e−018 C3  3.28422058e−018 C4 −6.06316417e−024 C4   1.50550421e−022 C4−1.50550421e−022 C5   6.11462814e−028 C5   0.00000000e+000 C5  0.00000000e+000 C6 −8.64346302e−032 C6   0.00000000e+000 C6  0.00000000e+000 C7   0.00000000e+000 C7   0.00000000e+000 C7  0.00000000e+000 C8   0.00000000e+000 C8   0.00000000e+000 C8  0.00000000e+000 C9   0.00000000e+000 C9   0.00000000e+000 C9  0.00000000e+000 Asphere of lens L805 Asphere of lens L806 Asphere oflens L811 K   0.0000 K   0.0000 K   0.0000 C1   2.42569449e−009 C1−6.74111232e−009 C1   2.28889624e−008 C2   3.96137865e−014 C2−2.57289693e−014 C2 −1.88390559e−014 C3 −2.47855149e−018 C3−2.81309020e−018 C3   2.86010656e−017 C4   7.95092779e−023 C4  6.70057831e−023 C4 −3.18575336e−021 C5   0.00000000e+000 C5  5.06272344e−028 C5   1.45886017e−025 C6   0.00000000e+000 C6−4.81282974e−032 C6 −1.08492931e−029 C7   0.00000000e+000 C7  0.00000000e+000 C7   0.00000000e+000 C8   0.00000000e+000 C8  0.00000000e+000 C8   0.00000000e+000 C9   0.00000000e+000 C9  0.00000000e+000 C9   0.00000000e+000 Asphere of lens L813 Asphere oflens L815 Asphere of lens L816 K   0.0000 K   0.0000 K   0.0000 C1  3.40212872e−008 C1 −3.15395039e−008 C1 −2.16574623e−008 C2−1.08008877e−012 C2   4.30010133e−012 C2 −6.67182801e−013 C3  4.33814531e−017 C3   3.11663337e−016 C3   4.46519932e−016 C4−7.40125614e−021 C4 −3.64089769e−020 C4 −3.71571535e−020 C5  5.66856812e−025 C5   1.06073268e−024 C5   0.00000000e+000 C6  0.00000000e+000 C6   0.00000000e+000 C6   0.00000000e+000 C7  0.00000000e+000 C7   0.00000000e+000 C7   0.00000000e+000 C8  0.00000000e+000 C8   0.00000000e+000 C8   0.00000000e+000 C9  0.00000000e+000 C9   0.00000000e+000 C9   0.00000000e+000 Asphere oflens L817 K   0.0000 C1   2.15121397e−008 C2 −1.65301726e−011 C3−5.03883747e−015 C4   1.03441815e−017 C5 −6.29122773e−021 C6  1.44097714e−024 C7   0.00000000e+000 C8   0.00000000e+000 C9  0.00000000e+000

1. A method of manufacturing an optical blank from a crystal material,wherein the crystal material has a crystallographic structure, andwherein the term lens means a lens or a lens part, said methodcomprising: determining an orientation of a first crystallographicdirection that is defined in relation to said crystallographicstructure; machining the crystal material into an optical blank so thatthe first crystallographic direction is substantially perpendicular to asurface of the optical blank; applying a marking to the optical blank ora mounting element for the optical blank, wherein the marking has adefined relationship to a second crystallographic direction which isoriented at a non-zero angle relative to the first crystallographicdirection.
 2. The method of claim 1, wherein the marking indicates areference direction that is perpendicular to the first crystallographicdirection, and wherein said reference direction is obtained as aprojection of the second crystallographic direction into a plane thatruns perpendicular to the first crystallographic direction.
 3. Themethod of claim 1, wherein determining the orientation of the firstcrystallographic direction comprises measuring a direction of a Braggreflex of a series of first crystallographic planes associated with thefirst crystallographic direction.
 4. The method of claim 3, wherein thestep of measuring the direction of the Bragg reflex is performed at aplurality of measurement positions of the optical blank, saidmeasurement positions being rotated in relation to each other about anaxis that extends perpendicular to said surface of the optical blank,and wherein the orientation of the first crystallographic direction isdetermined by comparing results obtained at said plurality ofmeasurement positions.
 5. The method of claim 1, wherein thecrystallographic structure comprises a crystallographic <100>-direction,directions equivalent to the <100> direction, a crystallographic<111>-direction, directions equivalent to the <111> direction, acrystallographic <110>-direction, and directions equivalent to the <110>direction.
 6. The method of claim 5, wherein the first crystallographicdirection coincides with a direction selected from the group consistingof the crystallographic <100>-direction, the directions equivalent tothe <100> direction, the crystallographic <111>-direction, thedirections equivalent to the <111> direction, the crystallographic<110>-direction, and the directions equivalent to the <110> direction.7. The method of claim 1, wherein the crystal material comprises calciumfluoride, strontium fluoride, or barium fluoride.
 8. The method of claim2, wherein the step of determining the reference direction is performedby measuring a direction of a Bragg reflex of a series of secondcrystallographic planes associated with the second crystallographicdirection.
 9. The method of claim 2, wherein the step of determining thereference direction is performed by a method that is known as Lauemethod.
 10. The method of claim 2, wherein a light ray incident on thelens is subject to a maximum optical path difference or a minimumoptical path difference for two mutually orthogonal states of linearpolarization if a projection of said light ray into a plane that isperpendicular to the first crystallographic direction is parallel to thereference direction.
 11. The method of claim 5, wherein the firstcrystallographic direction is a direction selected from the groupconsisting of the crystallographic <100>-direction, the directionsequivalent to the <100> direction, the crystallographic <111>-direction,the directions equivalent to the <111> direction, and wherein aprojection of the second crystallographic direction into a plane thatruns perpendicular to the first crystallographic direction is parallelto the projection of the crystallographic <110>-direction or one of thedirections equivalent to the <110> direction into a plane that runsperpendicular to the first crystallographic direction.
 12. The method ofclaim 5, wherein the crystallographic structure further comprises acrystallographic <331>-direction and directions equivalent to the <331>direction, wherein the first crystallographic direction coincides with adirection from a group that consists of the crystallographic<111>-direction and the directions equivalent to the <111> direction,and wherein the second crystallographic direction coincides with adirection selected from the group consisting of the crystallographic<331>-direction and directions equivalent to the <331> direction. 13.The method of claim 5, wherein the crystallographic structure furthercomprises a crystallographic <511>-direction and directions equivalentto the <511> direction, wherein the first crystallographic directioncoincides with a direction from the group that consists of thecrystallographic <100>-direction and the directions equivalent to the<100> direction, and wherein the second crystallographic directioncoincides with a direction selected from the group consisting of thecrystallographic <511>-direction and the directions equivalent to the<511> direction.
 14. The method of claim 3, further comprising the stepof removing portions of the crystal material from the optical blank thatwere traversed by a Bragg test radiation.
 15. The method of claim 8,further comprising the step of removing portions of the crystal materialfrom the optical blank that were traversed by a Bragg test radiation.16. An optical blank made from a crystal material and used inmanufacturing a lens for an objective, wherein the term lens means alens or a lens part, wherein the optical blank has a surface extendingsubstantially perpendicular to a first crystallographic direction; andcomprises: a marking affixed to the optical blank or a mounting elementof the optical blank, wherein the marking has a defined relationship toa second crystallographic direction which is oriented at a non-zeroangle relative to the first crystallographic direction.
 17. The opticalblank of claim 16, wherein the marking indicates a reference directionthat is perpendicular to the first crystallographic direction, andwherein said reference direction is obtained as a projection of thesecond crystallographic direction into a plane that runs perpendicularto the first crystallographic direction.
 18. An optical blank madeaccording to the method of claim
 1. 19. A method of making a lens fromthe optical blank of claim 16, wherein the lens has a lens axis, andwherein the method comprises: shaping the lens in such a way that thelens axis is substantially parallel to the first crystallographicdirection.
 20. The method of claim 19, further comprising: determiningan angular deviation between the lens axis and the firstcrystallographic direction; determining a direction of the angulardeviation, said direction of the angular deviation being perpendicularto the lens axis and being obtained by projecting the firstcrystallographic direction into a plane that extends perpendicular tothe lens axis; and marking the direction of the angular deviation on oneof the lens and a mounting element of the lens; or determining adirection angle between the reference direction and the direction of theangular deviation and assigning said direction angle to the lens.
 21. Amethod of making a lens from a crystal material with a crystallographicstructure, the term lens meaning a lens or a lens part, wherein themethod comprises: shaping the lens in such a way that a firstcrystallographic direction that is defined in the crystal structure issubstantially parallel to the lens axis; and applying a marking to thelens or a mounting element of the lens, wherein the marking has adefined relationship to a second crystallographic direction which isoriented at a non-zero angle relative to the first crystallographicdirection.
 22. The method of claim 21, wherein the marking indicates areference direction that is perpendicular to the lens axis, and whereinsaid reference direction is obtained as a projection of the secondcrystallographic direction into a plane that runs perpendicular to thelens axis.
 23. The method of claim 21, wherein the crystallographicstructure comprises a crystallographic <100>-direction, directionsequivalent to the <100> direction, a crystallographic <111>-direction,directions equivalent to the <111> direction, a crystallographic<110>-direction, and directions equivalent to the <110> direction. 24.The method of claim 23, wherein the first crystallographic directioncoincides with a direction selected from the group consisting of thecrystallographic <100>-direction, the directions equivalent to the <100>direction, the crystallographic <111>-direction, the directionsequivalent to the <111> direction, the crystallographic <110>-direction,and the directions equivalent to the <110> direction.
 25. The method ofclaim 21, wherein the crystal material comprises calcium fluoride,strontium fluoride, or barium fluoride.
 26. The method of claim 21,wherein the step of determining the reference direction is performed bymeasuring a direction of a Bragg reflex of a series of secondcrystallographic planes associated with the second crystallographicdirection.
 27. The method of claim 21, wherein the step of determiningthe reference direction is performed by a method that is known as Lauemethod.
 28. The method of claim 21, wherein a light ray incident on thelens is subject to a maximum optical path difference or a minimumoptical path difference for two mutually orthogonal states of linearpolarization if a projection of said light ray into a plane that isperpendicular to the first crystallographic direction is parallel to thereference direction.
 29. The method of claim 23, wherein the firstcrystallographic direction is a direction selected from the groupconsisting of the crystallographic <100>-direction, the directionsequivalent to the <100> direction, the crystallographic <111>-direction,the directions equivalent to the <111> direction, and wherein aprojection of the second crystallographic direction into a plane thatruns perpendicular to the first crystallographic direction is parallelto the projection of the crystallographic <110>-direction or one of thedirections equivalent to the <110> direction into a plane that runsperpendicular to the first crystallographic direction.
 30. The method ofclaim 26, further comprising the step of removing portions of thecrystal material from the optical blank that were traversed by a Braggtest radiation.
 31. The method of claim 21 further comprising:determining an angular deviation between the lens axis and the firstcrystallographic direction; determining a direction of the angulardeviation, said direction of the angular deviation being perpendicularto the lens axis and being obtained by projecting the firstcrystallographic direction into a plane that extends perpendicular tothe lens axis; and marking the direction of the angular deviation on oneof the lens and a mounting element of the lens; or determining adirection angle between the reference direction and the direction of theangular deviation and assigning said direction angle to the lens.
 32. Alens made from a crystal material for an objective, wherein the termlens means a lens or a lens part, wherein the lens has a lens axisoriented substantially in a first crystallographic direction, whereinthe lens comprises: a marking that is affixed to the lens or a mountingelement of the lens, wherein the marking has a defined relationship to asecond crystallographic direction which is oriented at a non-zero anglerelative to the first crystallographic direction.
 33. The lens of claim32, wherein the marking indicates a reference direction that isperpendicular to the first crystallographic direction, and wherein saidreference direction is obtained as a projection of the secondcrystallographic direction into a plane that runs perpendicular to thefirst crystallographic direction.
 34. A lens made according to themethod of claim
 21. 35. The lens of claim 32, wherein the lens comprisesa further marking, said further marking being affixed to the lens or themounting element of the lens, and wherein the further marking indicatesa direction of an angular deviation of the lens axis, said direction ofthe angular deviation being perpendicular to the lens axis and beingobtained by projecting the first crystallographic direction into a planethat extends perpendicular to the lens axis.
 36. A lens made from acrystal material for an objective, wherein the term lens means a lens ora lens part, wherein the lens has a lens axis oriented substantially ina first crystallographic direction, wherein the lens comprises: amarking that is affixed to the lens or a mounting element of the lens,wherein the marking indicates a direction of an angular deviation of thelens axis, said direction of the angular deviation being perpendicularto the lens axis and being obtained by projecting the firstcrystallographic direction into a plane that extends perpendicular tothe lens axis.
 37. An objective comprising the lens according to claim32.
 38. An objective with at least two lenses according to claim 37,wherein the at least two lenses are arranged with a rotation relative toeach other about their respective lens axes such that the referencedirections of any two of the at least two lenses are oriented at apredetermined angle of rotation relative to each other.
 39. Theobjective of claim 38, wherein the at least two lenses are made of afluoride crystal material with a cubic crystallographic structure, andwherein the angles of rotation are determined on the basis ofbirefringent properties of the fluoride crystal.
 40. The objective ofclaim 39, wherein the respective lens axis and the respective firstcrystallographic direction of each of the at least two lenses aremisaligned relative to each other by an angular deviation, and whereinwhen determining the angles of rotation, the respective angulardeviations of each of the at least two lenses are taken into account.41. An objective comprising at least two lenses according to claim 36,wherein the at least two lenses are arranged with a rotation relative toeach other about their respective lens axes such that the respectivedirections of the angular deviations of any two of the at least twolenses are oriented at a predetermined angle of rotation relative toeach other.
 42. The objective of claim 41, wherein the respectiveangular deviation of each of the at least two lenses causes an imageaberration of the objective, and wherein the angles of rotation betweenthe at least two lenses are determined so that the image aberrationssubstantially compensate each other.
 43. A microlithography projectionsystem comprising an illumination system serving to illuminate astructure-carrying mask; and an objective according to claim 37 servingto project an image of the structure-carrying mask onto alight-sensitive substrate.
 44. A method of manufacturing semiconductorcomponents with the microlithography projection system of claim 43.